OpenGL Texture Mapping

April 16, 2007

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Basic Stragegy

• Three steps to applying a texture1. specify the texture

• read or generate image• assign to texture• enable texturing

2. assign texture coordinates to vertices• Proper mapping function is left to application

3. specify texture parameters• wrapping, filtering

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Texture Mapping

s

t

x

y

z

image

geometry screen

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Texture Example

• The texture (below) is a 256 x 256 image that has been mapped to a rectangular polygon which is viewed in perspective

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• Define a texture image from an array of texels (texture elements) in CPU memory Glubyte my_texels[512][512][4];

• Define as any other pixel map Scan

Via application code

• Enable texture mapping­glEnable(GL_TEXTURE_2D) OpenGL supports 1-4 dimensional texture maps

Specify Texture Image

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Define Image as a Texture

glTexImage2D( target, level, components, w, h, border, format, type, texels );

target: type of texture, e.g. GL_TEXTURE_2Dlevel: used for mipmapping (discussed later)components: elements per texelw, h: width and height of texels in pixelsborder: used for smoothing (discussed later)format and type: describe texelstexels: pointer to texel array

glTexImage2D(GL_TEXTURE_2D, 0, 3, 512, 512, 0, GL_RGB, GL_UNSIGNED_BYTE, my_texels);

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Converting A Texture Image

• OpenGL requires texture dimensions to be powers of 2

• If dimensions of image are not powers of 2• gluScaleImage( format, w_in, h_in, type_in, *data_in, w_out, h_out,

type_out, *data_out );­data_in is source image­data_out is for destination image

• Image interpolated and filtered during scaling

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• Based on parametric texture coordinates• glTexCoord*() specified at each vertex

s

t 1, 10, 1

0, 0 1, 0

(s, t) = (0.2, 0.8)

(0.4, 0.2)

(0.8, 0.4)

A

B C

a

bc

Texture Space Object Space

Mapping a Texture

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Typical Code

glBegin(GL_POLYGON);glColor3f(r0, g0, b0);glNormal3f(u0, v0, w0);glTexCoord2f(s0, t0);glVertex3f(x0, y0, z0);glColor3f(r1, g1, b1);glNormal3f(u1, v1, w1);glTexCoord2f(s1, t1);glVertex3f(x1, y1, z1);

.

.glEnd();

Note that we can use vertex arrays to increase efficiency

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Interpolation

OpenGL uses bilinear interpolation to find proper texels from specified texture coordinates

Can be distortions

good selectionof tex coordinates

poor selectionof tex coordinates

texture stretchedover trapezoid showing effects of bilinear interpolation

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Texture Parameters

• OpenGL a variety of parameter that determine how texture is applied

Wrapping parameters determine what happens of s and t are outside the (0,1) range

Filter modes allow us to use area averaging instead of point samples

Mipmapping allows us to use textures at multiple resolutions

Environment parameters determine how texture mapping interacts with shading

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Wrapping Mode

Clamping: if s,t > 1 use 1, if s,t <0 use 0Wrapping: use s,t modulo 1

glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP )

glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT )

texture

s

t

GL_CLAMPwrapping

GL_REPEATwrapping

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Magnification and Minification

Texture Polygon

Magnification Minification

PolygonTexture

More than one texel can cover a pixel (minification) ormore than one pixel can cover a texel (magnification)

Can use point sampling (nearest texel) or linear filtering( 2 x 2 filter) to obtain texture values

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Filter Modes

Modes determined by­glTexParameteri( target, type, mode )

glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MAG_FILTER, GL_NEAREST);

glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MIN_FILTER, GL_LINEAR);

Note that linear filtering requires a border of an extra texel for filtering at edges (border = 1)

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Mipmapped Textures

• Mipmapping allows for prefiltered texture maps of decreasing resolutions

• Lessens interpolation errors for smaller textured objects

• Declare mipmap level during texture definitionglTexImage2D( GL_TEXTURE_*D, level, … )

• GLU mipmap builder routines will build all the textures from a given imagegluBuild*DMipmaps( … )

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Example

pointsampling

mipmapped pointsampling

mipmapped linear filtering

linear filtering

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Texture Functions

• Controls how texture is applied• glTexEnv{fi}[v]( GL_TEXTURE_ENV, prop,

param ) • GL_TEXTURE_ENV_MODE modes

­ GL_MODULATE: modulates with computed shade­ GL_BLEND: blends with an environmental color­ GL_REPLACE: use only texture color­ GL(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);

• Set blend color with GL_TEXTURE_ENV_COLOR

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Perspective Correction Hint

• Texture coordinate and color interpolation either linearly in screen space

or using depth/perspective values (slower)

• Noticeable for polygons “on edge”• glHint( GL_PERSPECTIVE_CORRECTION_HINT, hint )

where hint is one of • GL_DONT_CARE• GL_NICEST• GL_FASTEST

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Generating Texture Coordinates

• OpenGL can generate texture coordinates automatically

glTexGen{ifd}[v]()• specify a plane

generate texture coordinates based upon distance from the plane

• generation modes­GL_OBJECT_LINEAR­GL_EYE_LINEAR ­GL_SPHERE_MAP (used for environmental maps)

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From Sec.2.4 of “Advanced Graphics Programming Using OpenGL” (electronic copy available in NTHU library) http://www.netLibrary.com/urlapi.asp?action=summary&v=1&bookid=130156

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Texture Objects

• Texture is part of the OpenGL state If we have different textures for different objects,

OpenGL will be moving large amounts data from processor memory to texture memory

• Recent versions of OpenGL have texture objects

one image per texture object

Texture memory can hold multiple texture objects

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Other Texture Features

• Environmental Maps Start with image of environment through a wide

angle lens • Can be either a real scanned image or an image created

in OpenGL t

Use this texture to generate a spherical map

Use automatic texture coordinate generation

• Multitexturing Apply a sequence of textures through cascaded

texture units